Fix Build: GCC 4.9 has std::make_unique but not __cplusplus >= 201402L

Summary: [Folly] Fix Build: GCC 4.9 has `std::make_unique` but not `__cplusplus >= 201402L`.

Reviewed By: @igorsugak

Differential Revision: D2528584

fb-gh-sync-id: 9ad0567429698fb1320ad4066ee06add31acbd57

folly/Memory.h

@@ -37,7 +37,8 @@ namespace folly {
  * @author Xu Ning (xning@fb.com)
  */
 
-#if __cplusplus >= 201402L
+#if __cplusplus >= 201402L || \
+    defined __cpp_lib_make_unique && __cpp_lib_make_unique >= 201304L
 
 /* using override */ using std::make_unique;